It is known that any solid which sublimates without decomposition at normal pressure or under reduced pressure can in principle be refined by sublimation at suitable temperature and pressure; in actuality, because of a low rate of sublimation and poor refining efficiency, sublimation refining has been applied only to a limited number of solids. However, sublimation refining is useful for refining solids which are difficult to refine by distillation or recrystallization and particularly useful for refining compounds which start to decompose in a high temperature region. Some of apparatuses for such sublimation refining are disclosed in JP6-263438 A and JP7-24205 A.
Apparatuses for sublimation refining are divided by shape into such types as vertical and horizontal or by process into such types as gas entraining and vacuum. A suitable combination of these types provides a variety of apparatuses for sublimation refining and a proper selection of apparatuses is made in consideration of the properties of the sublimable substance to be refined such as thermal stability, vapor pressure and ease of vaporization, the throughput and the yield and purity of the object substance.
In case the amount of a solid to be refined is relatively large, however, it is difficult to sublimate the solid in a short time by heating in any of the aforementioned conventional apparatuses for sublimation refining and the solid shows an increasing possibility of decomposing or changing in property in the meantime. Moreover, precise control of the temperature in the sublimating and collecting units within a certain range becomes difficult to exercise and, as a result, the solid not only decomposes or changes in property but also does not improve sufficiently in purity.
When a metallic material such as ferrous metal is used as a structural material in an apparatus for sublimation refining, it becomes a matter of concern that a sublimable substance or impurities contained therein may react with the metal or may change in quality by the catalytic action of the metal. The reaction product or the product with changed quality, if any, contaminates the refined substance. In particular, trace metals exert great influences on the properties of metal complexes and it is important to prevent contamination of the refined substance.